The activities of monoamine biosynthetic enzymes were measured in brain regions of several hypertensive rat models at various ages. The types of hypertensive rats were the spontaneously hypertensive rat (SHR) and a stroke-prone substrain of the SHR as well as DOCA-salt and renal hypertensive rats. The genetically hypertensive rats had significantly elevated blood pressures as compared to the Wistar-Kyoto control rat after 5 weeks of age. During the early development of hypertension in the SHR, the activities of tyrosine hydroxylase in the hypothalamus and corpus striatum and of dopamine-beta-hydroxylase in the hypothalamus and pons-medulla were significantly higher than in the control rats. Tryptophan-hydroxylase was also elevated in the hypothalamus in SHR. From 3 to 8 weeks of age there appeared to be a significant correlation between hypothalamic dopamine-beta-hydroxylase activity and blood pressure in the hypertensive rats. In contrast, the activities of tyrosine hydroxylase and dopamine-beta-hydroxylase were slightly decreased in the DOCA-salt and renal hypertensive rats. It is suggested that noradrenergic or adrenergic neurons in the hypothalamus may participate in the initiation of elevated blood pressure in the genetic, but not in the DOCA-salt or renal hypertensive rats.